What does the circuit board in a solar light do?

I have been looking at instructables about making a solar phone charger. It seems the easiest way for me, is to just get a solar light.........But that is not the parts I have! What does the circuit do? Can I use that circuit with my batteries and my panel?

Did you make this instructable?

Typically, the circuit (a) routes power from the photocell to the battery, and (b) senses whether the photocell is producing power and only turns on the bulb when it isn't (ie, at night).

These are only driving a single LED. They don't produce anywhere near enough power to charge a phone. You might be able to sacrifice a bunch of them to get something that would charge a phone... slowly. See some of the solar-charger instructables.